1. Field of the Invention
The present invention relates generally to a broadband wireless access communication system using a multihop relay scheme. More particularly, the present invention relates to an apparatus and a method for processing change of a preamble of a Relay Station (RS).
2. Description of the Related Art
A fourth generation (4G) communication system, which is a next-generation communication system, aims to provide services of various Quality of Service (QoS) levels at a data rate of about 100 Mbps. Particularly, 4G communication systems are advancing in order to guarantee mobility and QoS in Broadband Wireless Access (BWA) communication systems such as wireless Local Area Network (LAN) systems and wireless Metropolitan Area Network (MAN) systems. Representative examples include an Institute of Electrical and Electronics Engineers (IEEE) 802.16d communication system and an IEEE 802.16e communication system.
The IEEE 802.16d communication system and the IEEE 802.16e communication system adopt an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme for physical channels. The IEEE 802.16d communication system does not consider mobility of a Subscriber Station (SS) and assumes a single-cell structure. In contrast, the IEEE 802.16e communication system considers the mobility of the terminal. A mobile terminal is referred to herein as a Mobile Station (MS).
FIG. 1 illustrates a conventional IEEE 802.16e communication system.
The IEEE 802.16e communication system has a multi-cell structure. That is, the IEEE 802.16e communication system covers a cell 100 and a cell 150. The IEEE 802.16e communication system includes a Base Station (BS) 110 which manages the cell 100, a BS 140 which manages the cell 150, and MSs 111, 113, 130, 151, and 153. Between the BSs 110 and 140 and the MSs 111, 113, 130, 151 and 153, signals are transmitted and received according to the OFDM/OFDMA scheme. Of the MSs 111, 113, 130, 151 and 153, the MS 130 moves in a boundary between the cell 100 and the cell 150. That is, the MS 130 moves within a handover area. When the MS 130 moves into the cell 150 managed by the BS 140 while transmitting and receiving signals with the BS 110, its serving BS is changed from the BS 110 to the BS 140.
Since the signaling is performed between the fixed BS and the MS over the direct link as shown in FIG. 1, the IEEE 802.16e communication system may establish a radio communication link of high reliability between the BS and the MS. However, because of the fixed BS, there is a low flexibility in the wireless network configuration for the IEEE 802.16e communication system. Thus, in a radio environment under severe changes of traffic distribution or traffic requirement, the IEEE 802.16e communication system hardly provides for an efficient communication service.
To address these shortcomings, using a stationary or mobile Relay Station (RS) or the conventional MSs, multihop relay data transmission may be applied to a conventional wireless cellular communication system such as IEEE 802.16e communication system. The wireless multihop relay communication system may reconfigure the network by promptly handling the communication environment change and operate the entire radio network more efficiently. For example, the wireless multihop relay communication system may extend the cell service coverage area and increase the system capacity. If a channel between the BS and the MS is in poor condition, the wireless multihop relay communication system may provide the MS with a channel having a better condition by installing the RS between the BS and the MS and establishing a multihop relay path via the RS. Also, in a cell boundary having a poor communication condition from the BS, the multihop relay scheme may offer a high-speed data channel and extend the cell service coverage area.
FIG. 2 illustrates a conventional multihop relay broadband wireless communication system for service coverage area expansion of the BS.
The wireless multihop relay communication system of FIG. 2 has a multi-cell structure. That is, the wireless multihop relay communication system covers a cell 200 and a cell 240. The wireless multihop relay communication system includes a BS 210 which manages the cell 200, a BS 250 which manages the cell 240, MSs 211 and 213 in the cell 200, MSs 221 and 223 managed by the BS 210 in a coverage 230 outside the cell 200, an RS 220 which provides multihop relay paths between the BS 210 and the MSs 221 and 223 in the coverage 230, MSs 251, 253 and 255 in the cell 240, MSs 261 and 263 managed by the BS 250 in a coverage 270 outside the cell 240, and an RS 260 which provides multihop relay paths between the BS 250 and the MS 261 and 263 in the coverage 270. Between the BSs 210 and 250, the RSs 220 and 260, and the MSs 211, 213, 221, 223, 251, 253, 255, 261 and 263, signals are transmitted and received using the OFDM/OFDMA scheme.
FIG. 3 illustrates a conventional multihop relay broadband wireless communication system for increasing system capacity.
The wireless multihop relay communication system of FIG. 3 includes a BS 310, MSs 311, 313, 321, 323, 331 and 333, and RSs 320 and 330 which provide multihop relay paths between the BS 310 and the MSs 311, 313, 321, 323, 331 and 333. Between the BS 310, the RSs 320 and 330, and the MSs 311, 313, 321, 323, 331 and 333, signals are transmitted and received using the OFDM/OFDMA scheme. The BS 310 manages a cell 300. The MSs 311, 313, 321, 323, 331 and 333 and the RSs 320 and 333 within the coverage of the cell 300 may transmit and receive signals directly to and from the BS 310.
However, some MSs 321, 323, 331 and 333 near the boundary of the cell 300 are subject to a low Signal to Noise Ratio (SNR) of direct links between the BS 310 and the MSs 321, 323, 331 and 333. The RSs 320 and 330 can raise the effective transfer rate of the MSs and increase the system capacity by providing high-speed data transmission paths to the MSs 321, 323, 331 and 333.
In the multihop relay broadband wireless communication system of FIG. 2 or FIG. 3, the RSs 220, 260, 320 and 330 may be infrastructure RSs installed by a service provider and managed by the BSs 210, 250 and 310 which are aware of the existence of the RSs in advance, or client RSs which serve as SSs (or MSs) or RSs in some cases. The RSs 220, 260, 320, 330 may be fixed, nomadic, or mobile like the MS.
As described above, the RS which relays the communication of the MS and the BS may be mobile. A mobile RS may get out of the service coverage area of the BS or its upper RS. When the mobile RS moves into the service coverage area of a new upper RS or the neighboring BS, the MS or the lower RS linked to the mobile RS may perform handover together with the mobile RS.
Meanwhile, when the mobile RS enters the service coverage area of a new target node (RS or neighbor BS), a preamble used by the mobile RS may interfere with the service coverage area of the other neighbor BS or the other RS. In this case, the preamble of the mobile RS needs to be changed. When the preamble of the mobile RS is changed, the MS moving in the service coverage area of the mobile RS recognizes the mobile RS as a new node. Accordingly, the serving BS of the MS hands the MS over to the mobile RS which sends the changed preamble.
The serving BS sends a handover indication message, including a changed preamble index of the mobile RS, to the lower MSs of the mobile RS. For example, the handover indication message can be a MOB_BSHO-REQ message with a forced handover indicator set to ‘1’. The MSs receiving the handover indication message determine to hand over to the target node (the mobile RS) of the preamble index and conduct a network re-entry procedure to the target node. Herein, the network re-entry procedure includes a code based ranging (RNG-REQ/RSP), a basic capability negotiation (SBC-REQ/RSP), authentication (PKM-REQ/RSP), registration (REG-REQ/RSP), and so on.
As described above, when the preamble of the mobile RS is changed, the MS moving in the service coverage area of the mobile RS is forcibly handed over with the changed preamble. Thus, the node of the changed preamble may continue servicing the MS. However, when the MS not moving outside the service coverage area of the mobile RS unnecessarily conducts the handover (including the network re-entry procedure), unnecessary overhead is created. Thus, there is a need for a method for reducing the overhead when the preamble of the mobile RS is changed.